A gas distribution showerhead is located at the reactor chamber ceiling overlying the workpiece or semiconductor wafer. One conventional showerhead has an annular plenum in which gas is introduced at one end and circulates azimuthally around the annular plenum. The gas injection orifices of the showerhead are azimuthally distributed outlets in the floor of the plenum. One problem with such a showerhead is that gas distribution is azimuthally non-uniform because the gas pressure is not uniform along the azimuthally flow path through the plenum. Another problem is that during some process transitions, such as a transition from an Argon process gas to an Oxygen process gas, some arcing (plasma light-up) in the gas outlets occurs. This is due at least in part to the non-uniform distribution of Argon and Oxygen in the plenum during the transition. During the transition, Oxygen predominates in the region nearest the gas supply and Argon predominates in the region furthest from the gas supply.
The plasma below the showerhead has a corresponding non-uniform distribution of Oxygen and Argon. Plasma density becomes correspondingly non-uniform because the portion of the plasma containing more Argon absorbs more plasma source power. Moreover, the sheath thickness of the portion of the plasma containing more Argon is less than the portion containing predominantly Oxygen. This leads to light-up or arcing in the showerhead outlets overlying the region of the plasma containing more Argon than Oxygen. This condition may last until all the Argon has been displaced by the incoming Oxygen gas, which may take on the order of a few seconds.
There is a need to introduce process gas in a manner that avoids such non-uniform distribution of gases during a process transition from one process gas to a different process gas.